CT systems of the third generation type include an X-ray source and an X-ray detector system secured to diametrically opposed sides of an annular-shaped disk. The disk is rotatably mounted within a gantry support so that during a scan, the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system typically includes an array of detectors disposed as a single row in the shape of an arc of a circle having a center of curvature at the point, referred to as the "focal spot", where the radiation emanates from the X-ray source. The X-ray source and the array of detectors are positioned so that the X-ray paths between the source and each detector all lie in the same plane (hereinafter the "slice plane" or "scanning plane") which is normal to the rotation axis of the disk. Since the X-ray paths originate from what is substantially a point source and extend at different angles to the detectors, the X-ray paths resemble a fan, and thus the term "fan beam" is frequently used to describe all of the X-ray paths at any one instant of time. The X-rays incident on a single detector at a measuring instant during a scan are commonly referred to as a "ray", and each detector generates an output signal indicative of the intensity of its corresponding ray. Since each ray is partially attenuated by all the mass in its path, the output signal generated by each detector is representative of the density of all the mass disposed between that detector and the X-ray source (i.e., the density of the mass lying in the detector's corresponding ray path).
The output signals generated by the X-ray detectors are normally processed by a signal processing portion of the CT system. The signal processing portion generally includes a data acquisition system (DAS) which filters the output signals generated by the X-ray detectors to improve their signal-to-noise ratio. The filtered output signals generated by the DAS are commonly referred to as "raw data signals." The signal processing portion usually includes a projection filter which logarithmically processes the raw data signals to generate a set of projection data signals so that each projection data signal is representative of the log attenuation produced by the mass lying in a corresponding ray path. The collection of all the projection data signals at a measuring instant is commonly referred to as a "projection" or a "view." During a single scan, as the disk rotates, a plurality of projections are generated such that each projection is generated at a different angular position of the disk. The angular orientation of the disk corresponding to a particular projection is referred to as the "projection angle."
A CT image may be generated from all the projection data signals collected at each of the projection angles. A CT image is representative of the density of a two dimensional "slice," along the scanning plane, of the object being scanned. The process of generating a CT image from the projection data signals is commonly referred to as "filtered back projection" or "reconstruction," since the CT image may be thought of as being reconstructed from the projection data. The signal processing portion normally includes a back projector for generating the reconstructed CT images from the projection data signals.
One problem with CT systems is that a variety of noise and error sources may potentially contribute noise or artifacts to the reconstructed CT images. CT systems therefore typically employ a host of signal processing techniques to improve the signal-to-noise ratio and to reduce the presence of artifacts in the reconstructed CT images.
One important factor which can cause unwanted artifacts to appear in the reconstructed CT images relates to the uniformity and stability of the X-ray detectors. If the response of a single detector is out of calibration with respect to the other detectors in the array, the single detector will cause an artifact to appear in the reconstructed CT image having the appearance of a circular ring, or one or more circular arcs, centered about the "center" of the reconstructed CT image (where the "center" of the reconstructed CT image corresponds to the location of the rotation axis of the disk). If more than one detector is out of calibration, they collectively cause a group of concentric circular rings or circular arcs to appear in the reconstructed CT image. Such artifacts are typically referred to as "rings," and "deringing" or "ring suppression" refers to methods and apparatus for reducing or eliminating the appearance of rings in the reconstructed CT images.
Ideally, the X-ray detectors are constructed so that their transfer functions or responses are all equal. However, this is difficult to achieve in practice. In many CT systems, the signal processing portion contains response calibration tables which are used to adjust the projection data signals to compensate for differences in the detector responses and thereby suppress rings in the image. The response calibration tables are typically generated by scanning objects of known density and shape, often referred to as "phantoms." The response calibration tables are updated periodically.
An important factor which can contribute to artifacts such as rings in a CT image is a phenomenon known as beam hardening, which is the change in the mean energy of the X-ray beam as it passes through a subject. Most CT scanners employ a polychromatic spectrum of X-ray energies to image a subject. Since low-energy photons tend to be preferentially attenuated, the mean energy of an X-ray beam increases as it passes through a length of material. A consequence of this beam hardening is that the projection signals vary nonlinearly with material thickness. Near the center of an object, this spectral nonlinearity decreases the CT number of the image, resulting in an image that appears cupped. The effect also produces rings and bands in third-generation scanners since they are sensitive to small differential variations in detector gain.
Methods for correcting beam hardening artifacts can involve some form of calibration to compensate detector readings for spectral nonlinearity, post processing of measured data using deringing algorithms and/or dual-energy imaging. One method for performing the nonlinear calibration involves comparing the attenuation produced by a known length of some known water-like material to its ideal value. The water-like material is used to simulate the density of soft tissue. By building a look-up table with these measurements of the known material, subsequent attenuation measurements performed on an unknown subject can be compensated for the nonlinearity of the detector readings. Use of a single bulk calibration table corrects a set of projections on average but leaves small differential gain errors between detectors. These small differential errors can be on the order of a fraction of a percent. Without further correction, these gain errors produce ring and band artifacts in reconstructed images.
At least two CT manufacturers use cylindrical plastic or water-filled phantoms to calibrate detector readings for spectral nonlinearity. In one approach, described in, for example, U.S. Pat. No. 4,352,020, issued Sep. 28, 1992, entitled, "Method and Apparatus for Examining a Subject," precisely centered plastic phantoms are scanned using a rotating CT gantry. This technique allows an estimate of attenuation to be made, since multiple rotations of data can be averaged to form each projection. However, only a limited number of attenuation measurements are obtained, due to the four or five phantoms used in the calibration procedure. As a result, a complicated interpolation and extrapolation scheme is required to generate the calibration table. In addition, the precise phantom alignment required by the technique complicates the calibration procedure, making it more labor intensive and dependent on individual operator skill.
A variant of this centered phantom approach involves scanning a single cylindrical phantom positioned off-center with respect to the center of rotation of the scanner. Such an approach is described in, for example, U.S. Pat. No. 5,214,578, issued May 25, 1993, entitled, "Method and System for the Calibration of an X-ray Scanner Using an Off-Centered Circular Phantom." This approach allows each detector channel to see a wider range of attenuation values and eliminates the need for precise centering of the phantom. However, the technique requires a homogeneous phantom with a precisely known geometry. Use of a single phantom also limits the number and range of attenuation measurements acquired.
Another method, described in U.S. Pat. No. 5,774,519, issued Jun. 30, 1998, entitled "Method of an Apparatus for Calibration of CT Scanners," involves taking scans of off-centered cylindrical water phantoms. The technique reprojects images of the water phantoms to estimate the gain errors associated with a given attenuation. This provides a range of attenuation measurements, eliminates the need to precisely center the phantom and allows inexpensive calibration phantoms to be used. However, use of a water phantom limits the number and range of attenuation measurements obtained. Also, use of reprojection makes the algorithm more computationally expensive.